1. Field of the Invention
The present invention is generally related to to a method of producing patterned polyimide films using wet development of polyimide precursors through a photoresist mask. More particularly, the invention is concerned with the formation of low thermal coefficient of expansion (TCE) polyimide patterns starting with a polyamic acid precursor, typically, that derived from 3,3',4,4'-biphenyltetracarboxylic acid dianhydride-p-phenylenediamine (BPDA-PDA). The invention is further concerned with generating polyimide patterns with complete retention of the intrinsic properties of the polyimide backbone chemistry and formation of metallurgical patterns in low TCE polyimide dielectric.
2. Description of the Prior Art
High temperature stable polymers, particularly polyimides, are well-known in the microelectronics industry and have applications as insulators, interlevel dielectrics, and passivation layers for various types of metallurgy. Polyimides are preferred over inorganic insulators because they generally have a lower dielectric constant, are more amenable to film processing, and encompass a wide variety of chemistries that can be chosen to meet the functional requirements for different applications. However, a major drawback of conventional flexible chain polyimides, such as that derived from pyromellitic dianhydride-4,4'-oxydianiline (PMDA-ODA) polymer precursor which is formed by the condensation reaction of PMDA and ODA, is that they have a relatively high degree of in-plane thermal expansion. For example, the in-plane thermal expansion for PMDA-ODA polyimide is typically 33-35 ppm/degree at 100.degree. C., which contrasts sharply with the typical thermal expansion of about 3 ppm/degree for inorganic substrates such as silicon, silicon oxide, silicon nitride, and other ceramics.
Differences in thermal expansion between a polyimide film and an underlying substrate can lead to thermal stress problems such as interfacial delamination upon high temperature processing. There have been recent developments in the field of low TCE polyimides which may address these thermal stress problems by providing polyimides with thermal expansion comparable to inorganic substrates. U.S. Pat. No. 4,690,999 discloses an example of uniaxially oriented low TCE polyimides and composite shaped articles.
Use of a polyimide insulator or interlevel dielectric requires a suitable method of forming polyimide patterns corresponding to desired metallurgical circuits. Prior methods for patterning polyimides have included photoprocessing and wet etch techniques.
Rubner et al., Photographic Science and Engineering (SPSE), Vol. 23, at pages 303-309 (1979) describe an approach which uses photoreactive polymer precursors. In this approach, generally negative working photosensitive polyamic acid salts or ester derivatives carrying photopolymerizable acrylate based crosslinking groups are used. These reactive precursors are directly patterned prior to imidization using conventional photolithographic techniques involving imagewise exposure through a mask and solvent development. After pattern formation, the films are baked or otherwise cured to form thermally stable polyimide patterns. Most of the commercial photosensitive polyimide compositions currently available are based on this approach and utilize a wide variety of photosensitizers and crosslinking additives; however, they are limited to flexible chain, relatively high TCE polyimide patterns.
It is generally recognized that the photoreactive polymer precursor approach results in inferior functional properties of the final cured film as compared to the corresponding polyimide films formed from non-photoreactive precursors. In the case of low TCE polyimides formed with photoreactive precursors, it has been observed that the fully cured polyimide has inferior mechanical, electrical, and thermomechanical properties relative to the polymer obtained using non-photosensitive precursors. Moreover, commercially available photosensitive precursors generally provide only negative tone patterns, and the formulations typically have poor shelf life and are prone to rapid gellation during storage. In addition, the photosensitive precursor formulations typically have accompanying lithographic control problems due to depth-of-focus limitations when patterning thick films. Furthermore, patterned films undergo up to 50% shrinkage upon curing which results in severe deformation of image profile and the extent of this problem is further related to the pattern density.
Generation of polyimide patterns by wet etching the non-photosensitive precursors represents an attractive alternative to incorporating photoreactive units in the polymer chain. According to this method, a polyamic acid film is formed on a substrate such as a silicon wafer, partially baked to remove solvent, then overcoated with a positive photoresist such as Shipley AZ 1350J, AZ 4210, or a negative resist such as Kodak KTFR. When using positive resists such as those based on a diazonaphthoquinone-novolac systems, the resist is imagewise exposed and developed with an aqueous base, such as potassium hydroxide (KOH) or tetramethylammoniumhydroxide (TMAH). The underlying polymer is patterned during the same step of developing the resist. The resist is then stripped with an organic solvent, and the patterned underlying film is subsequently baked or otherwise cured to form the cured polyimide patterns. This approach has been used to pattern conventional polyamic acids such as PMDA-ODA; however, problems arise from the isotropic etching inherent in the wet etching technique. Specifically, the prior art wet etch technique results in significant lateral etch, and this limits resolution, e.g., 25 .mu.m wide features are formed in 10 .mu.m thick films resulting in 6-7 .mu.m thick polyimide film after full cure. Recently, efforts have been made to adjust the conventional wet etch process such that polyimides formed from flexible chain polyimide precursors have improved performance in terms of image profile and residue-free polyimide patterns.
U.S. Patent No. 4,353,778 to Fineman et al. discloses a method of etching polyimide whereby a partially cured, approximately 16 .mu.m thick PMDA-ODA polymer film on a silicon wafer or ceramic substrate is coated with 5 .mu.m KTFR, a negative photoresist which is commercially available from Kodak, prebaked, imagewise exposed, and the resist layer is developed in organic solvent developer followed by a 130.degree. C./15 min bake. The underlying exposed polymer film is etched in aqueous KOH at 50.degree. C., baked again at 200.degree. C., and etched a second time to clean out the erodable material from the pattern side walls. The photoresist is then stripped and the patterned film is thermally cured to complete imidization. When using a positive photoresist mask, a single etchant is used to pattern the resist and the polyimide. There is no information on the image resolution and the final thickness after the cure.
U.S. Pat. No. 4,411,735 to Belani discloses an etch process and compositions for a polymeric insulating layer of polyimide isoindoloquinazoline-dione. According to this method, a first level, partially cured polyimide layer is etched with aqueous aliphatic or aromatic amines or with an aqueous tetramethylammoniumhydroxide-n-methylpyrrolidone (TMAH-NMP) mixture using a negative photoresist mask pattern such as Waycoat IC resist (commercially available from Hunt Chemical Corp.), which itself is developed with an organic solvent such as xylene. The patterned stack is then subjected to a second partial cure at 200.degree. C. to insolubilize the polyimide underlayer. The resist is then stripped with organic solvent and the patterned polyimide layer is subjected to a final thermal cure. Positive resists such as Shipley 1350J or AZ1470 and other etchants such as KOH, NH.sub.4 OH, NaOH, LiOH may also be utilized. After etching, O.sub.2 plasma treatment is used for descumming or removal of residues. Image resolution is described as 3, 10 and 50 .mu.m openings, but thickness is not given. Based on the viscosity given for the polyimide precursor and the spin speed, the final cured thickness may be only about 1-1.5 .mu.m.
Low TCE polyimides, such as BPDA-PDA and related compounds, have a rigid-rod back-bone chemistry and are distinctly different from conventional, high TCE polyimides, such as PMDA-ODA, which are characterized by a high degree of chain flexibility. The inventors have found that the standard wet etch methods described above are not suitable for patterning low TCE polyimide precursors such as BPDA-PDA polyamic acid. Specifically, the conventional wet etch method cannot be used to achieve residue-free images in up to 10 .mu.m thick fully cured films, they result in unsuitable resolution for highly integrated circuitry, and they cannot be used for relatively anisotropic etching or producing somewhat tapered wall profiles. This may be due to differences in the solubility characteristics of rigid-rod versus flexible chain polyimides, or there may be other structural factors responsible for the observed differences in the behavior of the polyamic acid precursors of these two categories of polyimides under wet etch conditions. Heretofore, there has been no wet etch process which is suitable for generating low TCE polyimide patterns.